1. Field of the Invention
This invention relates to a process for producing an electrophotographic photosensitive member, and more particularly to a method of controlling the surface profile of an electrophotographic photosensitive member so as to obtain an electrophotographic photosensitive member having a good cleaning performance.
2. Description of the Related Art
As an electrophotographic photosensitive member, in view of advantages of low prices and high productivity, an organic electrophotographic photosensitive member has become popular, which is an electrophotographic photosensitive member having a support and provided thereon a photosensitive layer (organic photosensitive layer) making use of organic materials as photoconductive materials (such as a charge generating material and a charge transporting material). As the organic electrophotographic photosensitive member, in view of advantages such as a high sensitivity and a variety for material designing, an electrophotographic photosensitive member is prevalent which has a multi-layer type photosensitive layer having a charge generation layer containing a charge generating material and a charge transport layer containing a charge transporting material; the layers being superposed to form the photosensitive layer. The charge generating material may include photoconductive dyes and photoconductive pigments. The charge transporting material may include photoconductive polymers and photoconductive low-molecular weight compounds.
The electrophotographic photosensitive member is, in its image formation process, used under a repeated cycle of charging, exposure, development, transfer, cleaning and charge elimination. Especially in the cleaning step which removes toners remaining on the electrophotographic photosensitive member after the transfer step is an important step in order to obtain sharp images. As a method for this cleaning, what is common is a method in which a rubbery cleaning blade is brought into pressure contact with the electrophotographic photosensitive member to scrape off the toners.
However, a cleaning blade showing a good cleaning performance has so large frictional force as to tend to cause problems such as an increase in drive torque, slip-away of toners because of a very small vibration of the cleaning blade and further turn-over of the cleaning blade. In recent years, it is also taken as a problem that the cleaning performance is affected by toners having been made small-diameter and high-function taking account of a trend toward higher image quality.
As a method of overcoming the above problems, a method is proposed in which the area of contact between the photosensitive member surface and the cleaning blade is made small by roughening the photosensitive member surface appropriately, to lower the frictional force between them. For example, a method is disclosed in which drying conditions set when the photosensitive layer is formed are controlled to roughen the photosensitive layer surface in orange peel surface (see, e.g., Japanese Patent Application Laid-open No. S53-092133). This method has an advantage that any special investment for installation is basically unnecessary because the surface is roughened in a usual photosensitive layer formation step. On the other hand, this method is disadvantageous in that it requires many factors for control, such as the temperature, humidity and time to be set in drying, the uniformity of atmosphere, the type of solvents, and so forth.
A method is also known in which powder particles are previously added to the surface layer to provide a rough surface (see, e.g., Japanese Patent Application Laid-open No. S52-026226). However, in general, where a powder is added to the photosensitive member, only a few powders are available which are suited for photosensitive members in respect of the materials, dispersibility and liquid stability of powders. Moreover, such powder may adversely affect properties of photosensitive members depending on the amount of its addition, and hence there is not so high a degree of freedom for the addition of powder. This method also has a disadvantage that desired surface properties are achievable with difficulty because of the leveling effect that comes at the time of coating.
Against such surface roughening in the coating step, as a method by which the surface profile can more readily be controlled, e.g., as a mechanical surface roughening method, a method is disclosed in which the photosensitive member surface is polished by using a wire brush made of a metal (see, e.g., Japanese Patent Application Laid-open No. S57-094772). This method has a difficulty that, when the brush is continuously used, it is difficult to achieve its reproducibility because brush bristle ends may deteriorate or polish dust may adhere to the bristle ends.
As another mechanically surface-roughening method, a method is available in which the photosensitive member surface is polished with a filmy polishing material (see, e.g., Japanese Patent Application Laid-open No. H02-150850). In this method, a fresh surface of the filmy polishing material can always be used in the polishing in virtue of a film wind-up unit. This enables achievement of reproducibility of the surface-roughening. Although the filmy polishing material has a disadvantage that it involves a high cost, this method has hitherto been considered to be a simple and effective method. However, abrasion dust of the photosensitive layer because of the polishing of, i.e., mechanical destruction of the photosensitive layer surface, and also the film-origin polishing material, may come into question.
As still another mechanically surface-roughening method, a method is disclosed in which the peripheral surface of an electrophotographic photosensitive member is roughened by blasting (see, e.g., Japanese Patent Application Laid-open No. H02-150850). This method has an advantage that the size and type of abrasive grains and blasting conditions may be controlled to enable control of surface profile to a certain extent, but on the other hand may come into question from the viewpoint of productivity and cost.
More specifically, in the background art, the surfaces of electrophotographic photosensitive members can be roughened to a certain extent, and this has brought certain effects. However, under existing circumstances, how to process surface profile more finely and in a more controlled state has not been established toward further improvements in performance and productivity.
Meanwhile, against the foregoing mechanically surface-roughening method, as a method by which the surface profile can more finely be controlled in a non-destructive way, a method is disclosed in which a touch roll or stamper (stamping die) having an unevenness profile on its surface is brought into contact with the surface of an electrophotographic photosensitive member to carry out compression forming (see, e.g., Japanese Patent Application Laid-open No. 2001-066814). According to this patent publication, a touch roll made of SUS304 stainless steel and having a prismatic and wavy surface profile is brought into contact with an electrophotographic photosensitive member at a pressure of 2×10−4 N to form on the surface of the electrophotographic photosensitive member a wavy profile of, e.g., 5 μm in average pitch and 5 μm in average depth. Such a working example is disclosed therein. A working example is also disclosed in which a stamper on which a well type surface profile of 100 nm in average length per one side and 100 nm in average depth is formed at a pitch-to-pitch distance of 100 nm is used to process the surface of an electrophotographic photosensitive member by compression forming for 2 minutes at a pressure of 0.8 N. As the result, a well type surface profile of 70 nm in average length per one side and 30 nm in depth has been formed on the surface of the electrophotographic photosensitive member at a pitch-to-pitch distance of 120 nm, as so disclosed. It is also disclosed that the forming precision can be improved by heating the electrophotographic photosensitive member and the stamper at the time of such surface processing and that the surface processing pressure is set at 1 N or less in order to maintain the roundness of the electrophotographic photosensitive member.
Such a compression forming technique is a technique in which an embossing technique which is a method for the unevenness processing of the surfaces of resin products or the like as conventionally known in the art, or a nano-imprinting technique on which researches are energetically forwarded in recent years as a fine surface processing technique, is applied to electrophotographic photosensitive members.
In general, such conventional techniques in which the surfaces of resin films or molded resin products are subjected to unevenness surface processing are carried out through the following steps (see, e.g., Japanese Patent Application Laid-open No. 2004-288784).
(1) A resin product to be surface-processed is heated to glass transition temperature or higher temperature of the resin (the step of softening the resin so as to be readily thermally deformed); (2) a stamper (stamping die) is heated to glass transition temperature or higher temperature of the resin and this is brought into pressure contact with the resin (the step of making the resin enter the interior of a fine surface profile of the stamper); (3) after lapse of a stated period of time, the resin and the stamper are cooled to their glass transition temperature or lower temperature (the step of fixing the fine surface profile); and (4) the stamper is separated from the resin product.
The foregoing steps enable batch transfer of fine surface profiles in accordance with the area of the stamper, and various surface processing objects can individually be processed according to the steps (a batch system). In the case of sheetlike surface processing objects, surface profiles corresponding to the area of the stamper can repeatedly be transferred while the processing objects are moved (a step-and-repeat system). The steps of heating and cooling are very important in the above steps. If the heating is carried out at a low temperature, the surface profile may insufficiently be transferred. If the cooling is insufficiently carried out, the surface profile having been transferred may come out of shape. Such problems tend to arise, and hence detailed optimization is required in accordance with various properties of the resin.
Moreover, surface processing non-uniformity may come about because of non-uniform pressure and temperature within the area of the stamper, or it is necessary to apply pressure over the whole area of the stamper. Hence, under existing circumstances, there remain problems on apparatus construction because the pressure used must set high and also the steps of heating and cooling must be repeated and further productivity is poor. Accordingly, various measures and improvements have been attempted in order to solve such problems.
In addition, the surface processing objects are commonly supposed to be made of flat-platelike or flexible materials, whereas a surface processing object like a cylindrical electrophotographic photosensitive member in the present invention, having a curvature and requiring the surface processing of a several microns to tens of microns thick resin layer formed on a support having a small elastic deformation level and having a hardness, is difficult to process in a good precision for the contact between its surface and the stamper. Thus, it is supposed to be very difficult to attain the surface processing uniformity from the viewpoint of pressure uniformity within the area.
From the foregoing, under existing circumstances, there are many problems in the surface processing of cylindrical electrophotographic photosensitive members by the batch system and the step-and-repeat system.
Meanwhile, as a method in which the surface of a surface processing object is continuously unevenness-processed while the processing object is moved, a surface processing method of producing an embossed sheet is disclosed (see, e.g., Japanese Patent Applications Laid-open No. H08-118469 and No. H11-207913). In this method, it is common that, first, a processing object resin sheet is kept heated and softened, and this is continuously inserted and pressured between a pressure roll and a pattern roll (embossing roll) to transfer the latter's surface profile to the sheet, followed by the step of cooling to obtain an embossed article (a roll system). Here, it is usual to provide a temperature-conditioning mechanism between the pressure roll and the pattern roll to cool the sheet for the profile transfer by pressuring and simultaneously for the profile fixing. This method enables processing objects to be embossed continuously and in a good productivity along a series of the above flows, and is useful as a method for the surface profile processing chiefly on a film of tens of microns or more in thickness.
Here, in an attempt to employ the above surface processing method as a method of embossing the cylindrical electrophotographic photosensitive member with a stated surface profile on its peripheral surface, the following problem may arise. That is, the surface of the cylindrical electrophotographic photosensitive member is constituted of a continuous peripheral surface. Hence, in an attempt to process the whole peripheral surface, the region having been processed first may reach the vicinity of a nip formed by the pressure roll and the pattern roll, at a point of time where the surface processing is finally completed. As the result, it follows that the region having first been embossed with the surface profile is again heated, so that the surface profile may come out of shape. It is also very difficult that the heating and cooling at forward and backward zones, respectively, of the pressure surface processing region are temperature-controlled on such a continuous thin resin film formed on the support. Hence, this is considered not practical.
Further, as another method of continuously surface processing an object while it is moved, a production method is disclosed which is carried out by roll embossing, intended for unevenness micropattern surface processing of optical devices (see, e.g., Japanese Patent Application Laid-open No. 2002-214414). According to this method, a three-dimensional profile can be transferred in a good precision, to a thin resin film formed on a substrate, as so disclosed. Stated specifically, a flat platelike processing object is placed on a movable transfer stage, and the stage is moved while a roll-shaped forming material having a micropattern on its surface is pressured, whereby the surface profile is continuously transferred to the thin resin film formed on the substrate. It is described that the transfer stage and the roll-shaped forming material may be heated or a heater may be placed at backward and forward zones of pressuring so that the thin resin film can be heated and softened, to thereby improve pattern forming performance. Here, in the cylindrical electrophotographic photosensitive member in the present invention, in an attempt to employ the above surface processing method so as to process the whole peripheral surface uniformly and continuously, the following problems arise.
In such a case, a support which corresponds to the substrate and a photosensitive layer and a protective layer which correspond to thin resin films formed on the substrate are always heated by an external means. More specifically, as the problem has come about when the surface processing method of producing an embossed sheet is employed, the problem that a surface profile having been transferred first comes out of shape may arise because the regions before surface processing and after surface processing stand continuous. Especially where the transfer of a surface profile to the photosensitive layer used in the electrophotographic photosensitive member is taken into account, the problem that the surface profile may come out of shape tends to arise more remarkably in view of the fact that the layer contains a charge-transporting material in a large quantity, compared with common thermoplastic resins.
As discussed above, the employment of the compression forming techniques in electrophotographic photosensitive members is supposed to be very useful, but any production method therefor has not sufficiently been established, and, under existing circumstances, there remains room for further improvement.